By identifying patients at high risk of relapse, residual disease assessment promises to increase the cure rate of children with acute lymphoblastic leukemia (ALL). The quantity of residual, morphologically undetectable disease (minimal residual disease or MRD) present during therapy is an independent predictor of patient outcome. Although polymerase chain reaction assays can quantify MRD, simpler methods are needed for clinical application. In the previous funding period, simple automated fluorescence detection assays were developed for quantification of B- and T-cell antigen receptor gene rearrangements, which can be used as leukemia-specific markers in 90 percent of ALL cases. The studies now proposed will determine the utility of these assays for quantification of leukemic cells in clinical samples (Aim 1). Universally applicable ALL markers are also needed for the integration of MRD assays into clinical protocols. Our previous studies showed that the neural gene, R-cadherin, is ectopically expressed in ALL but not in normal hematopoietic cells. Expression of R-cadherin should serve as a marker of recurrent leukemic cells and as an indicator of treatment-refractory ALL. The proposed studies will determine whether expression of R-cadherin can identify patients at a high risk of relapse and evaluate the efficacy of therapy (Aim 2). The application of ultra-sensitive MRD assays can be used to address an emerging problem in the treatment of ALL. Current intensive therapy has caused a delay in the timing of hematologic relapse. Assessment of MRD at end of therapy should permit identification of those patients with greatest risk of post-therapy relapse (Aim 3). The development of new automated MRD assays and the identification of new markers of leukemia should improve residual disease evaluation and thereby improve risk-directed therapy selection for every child with ALL.